AbstractThis work reports and critically reviews failure mechanisms induced by metal‐GaAs interaction and contact degradation in low and medium power GaAs MESFETs in the framework of a comprehensive reliability evaluation test plan, performed mainly on commercially purchased devices manufactured by different technologies. The results show that, at least as regards contact degradation phenomena, these technologies have reached sufficient maturity, and significant reliability levels have been achieved even for the most severe application and environments. Devices coming from some suppliers still suffer from reliability problems. such as ‘sinking’ of Au‐based gate metallization into the active channel. Al electromigration, Al/GaAs interdiffusion enhanced by high contacts electromigration, surface metal migration and short circuiting of closely spaced electrodes on GaAs with a non‐suitable surface preparation and/or passivation. All these failure mechanisms have been identified by means of suitable microanalytical techniques, correlated with device electrical degradation and thoroughly discussed in this paper by comparison with results previously reported in the technical l